Air bags are inflatable bags that remain folded and out of sight in readiness for a frontal collision. During a collision, chemical reaction of a material typically containing sodium azide produces gaseous products which inflate the bag and interpose it between the driver and the steering wheel or the front seat occupant and the dashboard.
A sensor of the type to which this invention is applicable is marketed by Breed Automotive Corp. It has a metallic ball free to move in a sealed cylinder. Air flow around the ball causes a pressure differential. The pressure differential causes a force proportional to and opposite the relative velocity of the ball with respect to the cylinder. The proportionality of force to velocity makes the sensor an acceleration integrator that initiates a chemical reaction upon achievement of a predetermined velocity change. The variation of air viscosity with temperature is compensated by making the cylinder and ball of materials of different thermal expansion coefficients thereby causing the gap between the ball and cylinder to change with temperature as required to maintain the performance of the sensor over a wide temperature range. A firing mechanism for initiating combustion of a stab primer includes a pin propelled by a first spring into the stab primer. A release means for the firing mechanism includes a trigger lever in the form of a metal rod extending perpendicular to a pivoted shaft having a "D" shaped cross section. The trigger lever is moved by the ball to its firing position during a crash. A second spring urges the trigger lever away from the firing position thereby causing the trigger lever to maintain the ball in a normal or resting position during normal operation of the vehicle. The force the trigger lever applies to the ball modifies the pure velocity change character of the sensor so that the velocity change required for release of the firing pin increases with the duration of the deceleration pulse of the crash. Movement of the ball during a crash rotates the trigger lever until the flat portion of the D-shaft releases the firing pin which is thereupon driven by the spring into a stab primer. Ignition of the stab primer initiates combustion of larger amounts of propellent material that generates gas for filling the air bag. Two complete sensor and firing mechanisms are combined into one package to enhance reliability by providing two units each capable of initiating combustion of the propellant.
This sensor is expensive to manufacture and one reason for the high cost is the large number of complex parts and the precision required in the trigger mechanism. Great care is required to make each part so as to insure minimum friction and maximum precision during the life of the sensor.
A belleville spring is a washer shaped piece of elastic material bent so that in its relaxed shape its inner circumference is in a plane offset from the plane of its outer circumference. Under pressure applied to its inner circumference the spring flattens and, under greater pressure, its inner circumference moves to the opposite side of the outer circumference. When released it returns to its initial relaxed shape. A belleville spring has two characteristics that are used to advantage in this invention. The first is that it can exhibit a negative spring rate over much of its operating range. In a typical case the spring force will rise from zero as the spring is deformed from its relaxed shape to a peak force at a first deflection and then, as the spring is additionally deformed, diminish to a minimum of perhaps one fifth of the peak force at a second deflection before resuming its increase. A second characteristic is that the inner and outer circumference of a belleville spring can be made to high accuracy. The high accuracy obtainable at acceptable cost allows a design to be dependent upon achieving high accuracy for purposes such as accurately determining the velocity at which the sensor will be actuated.
A snap disk resembles a belleville spring without the inner hole. A snap disk is a piece of spring material that has a normal relaxed shape approximating a section of a sphere. In this shape its potential energy is at its lowest value. It is also in equilibrium, or will also remain indefinitely in a second shape in which it is dished in the opposite direction. In its second shape it has greater potential energy than in its normal relaxed shape and will energetically snap into its relaxed position when it is urged in that direction by force applied to its center.
A general object of this invention is to provide a crash sensor for automotive vehicles which also overcomes certain disadvantages of the prior art.